Are you still getting the 1 single post beep, or just no beep at all?
I just had to change motherboards around, and forgot to plug in the 4 prong plug up by the processor. After turning the system on, I got no post beep and nothing on screen. It took me a moment while scratching my head as to what the heck was going on. Then I remembered I had forgot to plug that one in. After plugging it in, it started up and ran like its supposed to.
If you are getting a single post beep, then you may need to reset CMOS (BIOS) settings. To get your screen to come back up. You might even have to re-seat your graphics card in the slot (meaning remove it from the slot and then put it back in, making sure it is plugged in all the way).
If none of this has any effect, I think the next step would be your power supply. How old is the system? How much dust accumulates in your system? When was the last time it was cleaned out?